同步通信卫星工作的基本原理如图所示．卫星通信系统由卫星和地球站两部分组成．卫星在空中把地球站发射上来的电磁波放大后再返送回另一地球站．地球站则是卫星系统与地面公众网的接口，地面用户通过地球站出入卫星系统形成链路．由于同步通信卫星在赤道上空3 600 km，它绕地球一周的时间恰好与地球自转一周的时间一致，从地面看上去如同静止不动一般．三颗相距120°的卫星就能覆盖整个赤道圆周，故卫星通信易于实现越洋和洲际通信．最适合卫星通信的频率为1 GHz～10 GHz频段，即微波频段．通信卫星的种类也越来越多，按服务区域划分，有全球、洲际区域和国内通信卫星．

    When I was in the seventh grade, I was a volunteer at a local hospital in my town.Most of the 1 I spent there was with Mr. Gillespie.He never had any visitors and nobody seemed to care about his 2.

    I spent many days there holding his hand and talking to him,3 anything that needed to be done.He became a close friend of mine,4 he responded with only an occasional squeeze(捏) of my 5. Mr. Gillespie was in a coma(昏迷).

    I left for a week to vacation with my parents, and when I came back, Mr. Gillespie was 6. I didn't have the7 to ask any of the nurses where he was, for fear that they might8 me he had died.

    Several 9 later, when I was a senior in high school, I was at the gas station when I noticed a familiar face.When I10 who it was, my eyes filled with tears.He was 11! I got up the nerve to ask him if his name was Mr. Gillespie. With an uncertain look 12 his face, he replied yes.I 13 how I knew him and that I had spent many days talking with him in the hospital.His eyes welled up with tears, and he gave me the warmest hug I had ever 14.

    He began to tell me that15 he lay there comatose(昏迷的), he could hear me talking to him and could 16 me holding his hand the whole time. Mr. Gillespie17 believed that it was my voice and 18 that had kept him alive.

    Although I haven't seen him since, he fills my heart with 19 every day.I know that I made a(n)20 between his life and his death.

克隆羊“多莉”有三个母亲，影响“多莉”长相的主要是（　　）